Method of producing liquid oxygen



Aug. 22,v 1950 w. DENNIS I v2,519,892 ls'mon 0F mowbmq LIQUID OXYGEN Filed Jan. 1 6, 1514s LIQUID 0x6 YGEN PRODUCT Mamie-- INVENTOR v Y 44/801 D8640.

} ATTORNEYS Patented Aug. 22, 1950 UNITED STATES PATENT OFFICE METHOD OF PRODUCING LIQUID OXYGEN Wolcott Dennis, Darien, Conn assignor to Air Reduction Company, Incorporated, New York, N. Y., a corporation of New York Application January 16, 1945, Serial No. 572,994

' 1 Claim.

This invention relates to the separation of the constituents of air by liquefaction and rectification and particularly to an improved method of recovering purified oxygen.

The procedure for separating oxygen from air by liquefaction and rectification is well known and has been practised commercially for many years. In such an operation it is necessary to compress the air to relatively high pressures. The necessary lubrication of the compressor introduces relatively minute amounts of oil and decomposition products thereof to the compressed air. Other impurities of a. similar nature may be present in the atmosphere and enter with the air introduced to the compressor.

Such impurities, which are generally of hydrocarbon origin, are not readily separable from the air or its constituent parts. The impurities, being non-vaporizable at the temperatures prevailing in a liquefaction and rectification system, tend to accumulate with liquid fractions in various parts of the system, and particularly in the pool of liquid oxygen which collects at the bottom of the rectifier. This undesirable condition has led to many attempts to eliminate the nonvanorizable impurities before they enter the rectiflcation. The suggested means intended to accomplish this purpose are both complicated and inefiicient. They are particularly unsatisfactory in small units, which are designed to'produce oxygen as required and without the need for skilled and constant attention to the operation.

It is the object of the present invention to provide an effective method of recovering purified oxygen liquid relatively free from non-volatile impurities from a simple rectification column, thus affording a source of relatively pure liquid oxygen at relatively low cost.

Other objects and advantages of the invention will be apparent as it is better understood by reference to the following specification and the accompanying drawing, in which an apparatus suitable for the practice of the invention is illustrated diagrammatically.

In accordance with the present invention, I prefer to operate with a simple column in which liquid air is separated into oxygen and an efliuent consisting of all of the nitrogen and some of the oxygen. While the operation of such a column is not efiicient from the standpoint of securing all of the oxygen in the air, it is particularly advantageous in relatively small units designed to be started whenever oxygen is required. Thus, while such a column will ordinarily recover only about 65% of the oxygen in the entering air, it can be operated easily to produce liquid oxygen free from vaporizable impurities. The non-volatile impurities which may be present in the air will pass through the column and accumulate in the liquid at the bottom thereof. I have discovered that liquid oxygen relatively free from such non-volatile impurities may be recovered from the rectification provided the liquid is withdrawn at a point from one to three trays above the boiling pool of liquid oxygen at the bottom of the column. Thus, if a portion of the liquid flowing through the column is withdrawn at the point indicated, a liquid oxygen product free from volatile and relatively free from nonvolatile impurities is obtained.

In carrying out the invention, air is compressed and cooled in any of the usual types of equipment involving compressors, inter-coolers and exchangers such as are commonly employed in liquefaction systems. The details of these devices form no part of the present invention and hence are not illustrated in the drawing. Referring to the drawing, the air at suitable pressure, for example upward to 20 atmospheres absolute, is introduced through a pipe 5 to a coil 6 which is disposed within a chamber 1 at the bottom of a rectification column 8. The liquid which accumulates irfthe chamber 1 is under a lower pressit than that of the entering air, so that the lat er is condensed to a liquid which flows through the pipe'ii, a pressure reducing valve ill and a pipe H to the top of the column 8.

The column 8 includes a plurality of trays I2 with the usual bubble caps l3. The liquid delivered through the pipe ll flows downwardly over the trays l2 and is subjected to rectification with vapors rising through the trays, with the result that the liquid is progressively enriched in oxygen while nitrogen and some of the oxygen passes upwardly and escapes as effluent or waste gas through a pipe H which may deliver it to the exchangers (not shown). The rectification may be controlled readily to ensure the production of an oxygen liquid free from volatile impurities and relatively free from non-volatile impurities on the lower trays of the column by discarding a substantial proportion of the oxygen, for example about 35% thereof, in the waste gas.

A portion of this oxygen liquid overflows through a pipe I5 into the chamber I surrounding the coil 6 and affords the refrigerating agent for the entering air. The resultin vaporization of the liquid oxygen causes pure oxygen vapor to pass upwardly through the bubble caps I3 in a diaphragm l6 separating the column 8 from the chamber I. These pure owgen vapors give up their heat to the descending colder liquid existing in the upper portion of column 8 and are in turn condensed to pure liquid oxygen which then flows downwardly through the column along with the liquid oxygen which has been simultaneously separated from the liquid air fed to the top of column I through pipe II. In a simple column the amount of oxygen vaporized from the pool and condensed within the column is from -8 times as great as the amount of liquid oxygen separated from the liquid air fed to such a column.

At a point somewhat above the diaphragm II and preferably from one to three trays above the diaphragm. a pipe I! controlled by a valve ll permits the withdrawal of a portion of the liquid oxygen flowing downwardly through the column. This liquid oxygen is relatively free from non-volatile impurities which pass downwardly through the column and accumulate in the pool of liquid in the receptacle I. The percentage of non-volatile impurities in the liquid on the trays adjacent the pipe I1 is necessarily much lower than in the liquid accumulated in the chamber 1 because it is diluted with pure oxygen condensed from vapors rising from the latter liquid. It is to be noted that since the drawing is diagrammatic, the number of trays shown therein is merely indicative of such trays which may be spaced in the usual relation and supplied in suitable number to efiect the desired result. The position of the pipe I! with respect to the diaphragm i6 is not definitely fixed, since the point of withdrawal can be varied within a fairly wide range. The proper point for withdrawal of the purified liquid oxygen can be determined readily Inasmuch as the non-volatile impurities accumulate in the liquid pool at the bottom of the column, it is desirable to withdraw a portion of this liquid from time to time. A purge is controlled by a valve 20 is provided for that purpose. By withdrawing some of the liquid, the accumulated, non-volatile impurities can be kept at a minimum without interrupting the functioning of the column and the production of liqold oxygen having a greatly reduced content of non-volatile impurities.

Various changes may be made in the details of the procedure and particularly in the apparatus employed without departing from the invention or sacrificing the advantages thereof.

I claim:

The method of recovering liquid oxygen from air which comprises establishin a liquefied body rich in oxygen in the bottom of a rectification column having a rectification zone in the upper portion thereof, passing air under a pressure high enough to liquefy it at the temperature of said liquefied oxygen-rich body into heat exchange therewith with concomitant liquefaction of the air and vaporization of a portion of the women of the liquefied body, rectifying the liquefied air in said rectification zone with oxygen vapors rising from said liquefied body with concomitant production of a gaseous efiiuent rich in nitrogen and an oxygen-rich liquid product, discharging the gaseous eiiiuent, withdrawing, as the sole normal oxygen product of the rectification, and before passage thereof from the rectification zone, a portion of the liquid product of the rectification from a lower portion of the rectification zone where the liquid product of the rectification is free from volatile and relatively free from non-volatile impurities, and delivering the remaining portion of the liquid product of the rectification to the oxygen-rich liquefied body in the bottom of the rectification column to replenish it.

WOLCO'IT DENNIS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,068,219 Claude July 22, 1913 1,880,981 Pollitzer et a1 Oct. 4, 1932 1,959,883 Van Nuys May 22, 1934 2,000,992 Schlitt May 14, 1935 2,287,137 Ross June 23, 1942 

